BOSTON, Massachusetts (CNN) -- Democratic presidential nominee John Kerry accused President Bush on Thursday of letting front groups "do his dirty work" in questioning his military service during the Vietnam War.

"The president keeps telling people he would never question my service to our country. Instead, he watches as a Republican-funded attack group does just that," Kerry told a firefighters' union conference in his hometown of Boston.

"Well, if he wants to have a debate about our service in Vietnam, here is my answer: Bring it on."

The group Swift Boat Veterans for Truth has aired the ads in question in the likely November battleground states of Ohio, Wisconsin and West Virginia.

"They're a front for the Bush campaign. And the fact that the president won't denounce them tells you everything you need to know -- he wants them to do his dirty work," Kerry told a cheering crowd.

Bush campaign spokesman Steve Schmidt emphasized that Bush has not criticized Kerry's military service in Vietnam.

"Senator Kerry knows President Bush has called his service in Vietnam noble ... and the Bush campaign has tried to have a debate about the future, not the past," Schmidt said.

In Crawford, Texas, White House press secretary Scott McClellan suggested that Democrats were guilty of the same practice -- watching as other groups hurl charges at the opposition.

McClellan noted that Bush has been the subject of several critical ads by outside groups. For example, the group MoveOn has spent millions of dollars on ads that are sharply critical of Bush.

"The president has condemned all of the ads by the shadowy groups," McClellan said. "We have called on Senator Kerry to join us in calling for an end to all the unregulated soft money activity that is going on in this campaign."

Vietnam revisited

In the swift boat commercials, former sailors accuse Kerry of lying to receive two of his five combat decorations, a Purple Heart and the Bronze Star, and criticize his anti-war activism after he returned home from Vietnam. He also received a Silver Star and two Purple Hearts for other actions.

The ad includes Larry Thurlow, who commanded one of five swift boats in the Mekong Delta during an incident March 13, 1969, for which Kerry was decorated and Thurlow himself earned a Bronze Star.

Thurlow told CNN that Kerry's boat fled after a mine crippled another boat and was not under enemy fire when he returned to rescue an Army officer knocked overboard by a second mine that detonated nearby.

But the Navy citation for Thurlow's Bronze Star said "all units began receiving enemy small arms and automatic weapons fire from the river banks."

And the Navy's letter awarding Kerry the Bronze Star states that Kerry exhibited "great personal courage under fire" in rescuing Army Green Beret Lt. Jim Rassmann, who recommended Kerry for the decoration and who has publicly disputed Thurlow's account of what happened that day.

Thurlow -- who said he has no contact with the Bush campaign -- told CNN Thursday he did not know he had even received a Bronze Star until after he left the Navy, and he had not seen the citation in 30 years.

He contended it was Kerry who wrote the after-action report that was the basis for the citation -- even though another officer, Lt. Cmdr. George Elliot, signed the document.

"I would concede his [Kerry's] signature's certainly not on it. You're right," Thurlow said. "The reason I'm sure John filed it is because the entire report has the 94 boat, John's boat, as the central figure in all the action."

John O'Neill, one of the founders of the group that ran the ad, said no bullet holes were found in any boat that took part in the flotilla. "Kerry's report that day describes something like the battle of Gettysburg," O'Neill said.

Kerry's campaign said none of the men in the Swift Boat Veterans for Truth commercial served in the same boat as the Democratic nominee. And it described Thurlow as "completely discredited."

Kerry became a prominent antiwar activist after returning from Vietnam, and his allegation that U.S. troops routinely committed atrocities still rankles some veterans.

He received a lukewarm response Wednesday at the annual meeting of the Veterans of Foreign Wars, where some veterans upset with Kerry's criticism of the war left the audience and at least two stood and turned their backs to him.

New Kerry ad

The Kerry campaign released its own ad Thursday in Ohio, West Virginia and Wisconsin that features Rassmann saying Kerry saved his life.

"All these Viet Cong were shooting at me," Rassmann says in the ad. "I expected I'd be shot. When he pulled me out of the river, he risked his life to save mine."

A statement from the Kerry campaign said Rassmann is a registered Republican.

Swift Boat Veterans for Truth said late Thursday it planned to air a new ad, but no details were released.

The group is an independent "527" committee, named for the section of the tax code that governs them. The groups are forbidden to coordinate their operations with political parties or campaigns.

Similar groups have targeted Bush for criticism, and one of the most prominent, MoveOn, has run an ad accusing Bush of using family connections to get into the Air National Guard during Vietnam, then failing to report for duty.

"This election is about character," the ad's announcer intones. "It's about John Kerry, who left no man behind -- and George Bush, who simply left."

The Bush campaign says the president fulfilled his Guard obligations and was discharged honorably.